Preparation and photodetection performance oftwo-dimensional In<sub>2/3</sub>PSe<sub>3</sub> nanosheets

Two-dimensional layered semiconductors have been the research focus of electronics and optoelectronics owing to their atomic thickness, light-matter interaction, and large-scale integration. Metal phosphorus trichalcogenides (MPTs), possessing the structure of [P2S6]4– or [P2Se6]4– surrounded with metal cations, have emerged as newly rising stars and drawn widespread interests of researchers due to their novel electrical and optical properties. Compared with common transition metal dichalcogenides, two-dimensional (2D) MPTs have larger optical bandgaps (from 1.3 to 3.5 eV), higher light absorption efficiency and richer bandgap structures due to the increased degree of freedom from the elements and atomic ratio, which highlight its great advantages in the field of optoelectronics. Among MPTs, In2/3PSe3 owns special structure, in which two-thirds of the octahedral holes are occupied by the indium ions and the other one-third are vacant. The unique structure endows In2/3PSe3 with intrinsic broken-symmetry, showing potential in non-linear optics. It was reported that bulk In2/3PSe3 is a direct-bandgap semiconductor (2.0 eV), in which the electron transition process from the conduction band to valence band is direct without phonon assisting, indicating great potential in application of optoelectronic devices. However, there is no research reported on the optoelectronic property of In2/3PSe3. According to the promising properties mentioned above, it is extremely urgent to characterize the optical and optoelectronic properties of 2D In2/3PSe3 nanosheets. In this work, high-quality In2/3PSe3 single crystals are synthesized by chemical vapor transport method and for the first time we achieve 2D In2/3PSe3 nanosheets on SiO2/Si substrate by mechanical exfoliation. X-ray diffractometer (XRD) and transmission electron microscope (TEM) are performed on In2/3PSe3 nanosheets, which demonstrate their high quality and single-crystalline nature. Considering that the non-centrosymmetric space group R 3, we further explored the SHG properties of In2/3PSe3 nanosheets. The intrinsic SHG response further verifies the broken-symmetry nature of In2/3PSe3. In addition, photoluminescence spectra of In2/3PSe3 nanosheets have revealed the exciton-like transition mechanism and longer PL lifetime than previously reported 2D materials, which may own to the high crystalline and low defects of as-synthesized crystals. To investigate the optoelectronic behavior of 2D In2/3PSe3 nanosheets, two-electrodes photodetectors based on In2/3PSe3 nanosheets are fabricated. Impressively, the In2/3PSe3 nanosheets-based photodetector exhibits an ultra-low dark current (25 fA), excellent detectivity (6.28 ´ 1011 Jones), high on/off ratio (4 ´ 104), and ultra-fast response rate (14 μs/24 μs) under 365 nm illumination, which are superior to other typical 2D ultraviolet photodetectors. The main photocurrent generation mechanism of the photodetector is photoconductive effect. The photogenerated free electrons and holes are separated rapidly to the opposite electrodes without trapping in defect states, leading to the ultra-fast response. These excellent optoelectronic behaviors of In2/3PSe3 nanosheet suggest its appealing prospect in future application of photodetection.